Osteoclastic resorption of calcium phosphates is potentiated in postosteogenic culture conditions

Cell-mediated resorption of densely sintered hydroxyapatite (HA1250), tricalcium phosphate (TCP), and 600 degrees or 900 degrees C calcined hydroxyapatite (HA600 and HA900, respectively), was investigated by using two culture systems. The first was an osteoclastic cell culture, and the second was a two-stage culture that was composed of a bonelike tissue formation on the substrata in the first stage and its subsequent resorption by osteoclasts in the second stage. Neither of the materials showed resorption or surface alterations in the osteoclastic cell culture, except for some limited phagocytotic activity on HA600 and HA900. In the two-stage culture, production of mineralized extracellular matrix was only observed on HA1250 and TCP, and its subsequent resorption by osteoclastlike cells was evident. Small and occasionally larger tartrate-resistant acid phosphatase positive cells produced 20-150 microns diameter resorption pits in both the mineralized extracellular matrix on HA1250 and TCP and the surfaces of HA600 and HA900. Resorption of the mineralized extracellular matrix on TCP also resulted in degradation of the underlying ceramic surface, mainly initiating from intergrain boundaries, whereas the surface of HA1250 remained unaltered. The results of this study clearly demonstrate that osteoclastic resorption of calcium phosphates is potentiated in postosteogenic culture conditions. A possible role for bone matrix constituents in cell-mediated resorption is hypothesized, whereas the occurrence of resorption seems to be mainly governed by the combined effects of material characteristics such as grain size and crystal structure.